FRAM (Ferroelectric RAM) is random access memory that combines the fast read and write access of dynamic RAM (DRAM)—the most common kind of personal computer memory—with the ability to retain data when power is turned off (as do other non-volatile memory devices such as ROM and flash memory). Because FRAM is not as dense (i.e., cannot store as much data in the same space) as DRAM and SRAM, it is not likely to replace these technologies. However, because it is fast memory with a very low power requirement, it is expected to have many applications in small consumer devices such as personal digital assistants (PDAs), handheld phones, power meters, and smart card, and in security systems. FRAM is faster than flash memory. It is also expected to replace EEPROM and SRAM for some applications and to become a key component in future wireless products.
As depicted in FIG. 2, a ferroelectric memory cell typically comprises a ferroelectric capacitor 9 and a selection transistor 2. The ferroelectric capacitor 9 comprises a stack of a conductive bottom electrode stack 10, a ferroelectric film 11, and a conductive top electrode 12. The ferroelectric memory cell is programmed by applying an electrical signal to the conductive top and bottom electrodes across the ferroelectric film 11. When an electric field is applied to a ferroelectric crystal, the central atom of the ferroelectric compound moves in the direction of the field. Internal circuits sense the charge required to move the atom. When the electric field is removed from the crystal, the central atom stays in position, preserving the state of the memory.
The formation of a crystalline ferroelectric film typically requires high temperature treatment in oxygen ambient. The film can be prepared by different techniques, such as spin-on, physical vapor deposition (PVD), chemical vapor deposition (CVD), and metal organic chemical vapor deposition (MOCVD). MOCVD may be performed in a two-step process, wherein in a first step the ferroelectric film is deposited at lower temperature, and afterwards in a second step the ferroelectric film is crystallized at a higher temperature, e.g. a temperature higher than 400° C. in an oxygen ambient. Alternatively, MOCVD may be performed in a one-step process at a higher temperature in oxygen ambient, wherein deposition and crystallization of the ferroelectric film occur simultaneously.
Examples of ferroelectric materials include, but are not limited to SrBi2Ta2O9 (SBT), Pb(Zr,Ti)O3 (PZT) and (Bi,La)4Ti3O12 (BLT). All ferroelectric layers ultimately incorporate oxygen. This oxygen is a part of the so-called “perovskite” crystal structure, which is typical for ferroelectric films.
Typically, very complex bottom electrode-barrier structures are used to avoid oxygen diffusion to the plug during the processing of the ferroelectric layer, as in a structure identified by layers in the order Pt/IrO2/Ir(/TiN) wherein TiN is an additional layer which protects the contact plug from interaction with the electrode stack or improves adhesion. See, e.g., D. Jung et al., Technical Digest IEDM (International-Electron Devices Meeting), San Francisco, Calif, Dec. 10-13, 2000, page 00-801, paper 34.4.1. This TiN layer can be part of the contact or can be formed on top of the contact.
The large number of processing steps required to fabricate a complex bottom electrode structure, besides being cost inefficient and environmentally unfriendly, imposes stringent requirements on the fabrication of stacked ferroelectric memory cells.